1. Field of the Invention
The present invention relates to a centralized remote-supervisory system and a data acquisition method thereof, wherein the system comprises a central processing unit and a large number of terminal units, each terminal unit having a plurality of sensors. And more particularly, the present invention relates to an improved system and a method of efficient data collection in a centralized remote-supervisory system, wherein the system is expected to work under the different conditions such that detected data by the sensor shows a rapid change during a specific time zone and that detected data shows a comparatively slow change during a normal time zone other than the specific time zone. The system of the present invention is particularly useful for controlling environment in a building, operation of a dam-type electric power station, an electric power self-supporting system at power stoppage, etc.
2. Description of the Related Art
FIG. 1 shows a concept of a centralized remote-supervisory system. A central processing unit 1 comprises a data communication unit 1a connected with a bus line 3. The bus line 3 is further connected to a plurality of terminal units 2 which are located far distant away from the central processing unit 1. Thus the system of FIG. 1 constitutes the centralized remote-supervisory system.
Each terminal unit 2, for example, comprises a CPU 2a (processing unit in the terminal unit is abbreviated as CPU 2a) and a memory 2b, a timer 2c and an I/O (input/output) interface unit 2d. A plurality of sensors 4 for detecting such as temperature, humidity, pressure, flow rate, etc. are connected to the I/O interface unit 2d. The detected data by the sensor 4 are periodically stored in the memory 2b under the control of the CPU 2a and the timer 2c.
In the prior art method utilizing the above system, the timer 2c gives periodically an interruption command to the CPU 2a. The CPU 2a when the interruption command is received, reads the stored data, and transmits the data to the central processing unit 1. The transmitted data from the terminal unit 2 are received at the central processing unit 1 and subjected to data processing such as an averaging process and the like, and finally utilized for controlling the associated equipments or machines with this system.
In the prior art method, the data are transmitted to the central processing unit 1 at regular periods, there is a problem that the load to the central processing unit 1 for data processing increases with an increase of the numbers of terminal units 2 and sensors 4. In order to cope with this problem, a memory area is further provided for storing a threshold level .DELTA.Q in the memory 2b. The threshold level .DELTA.Q is a predetermined constant and stored in advance in the terminal unit 2. The periodically detected data by the sensor 4 is once stored in the memory 2b and the next detected data Dc (current data) by the same sensor is compared with the previously detected and stored data Dp. If an absolute value .DELTA.D of the differential (=Dc-Dp) between these two data is larger than the predetermined threshold level .DELTA.Q, the differential data .DELTA.D is judged to be appreciable, and the data Dc is transmitted to the central processing unit 1.
FIG. 2 is a sequence flow chart of the above data collection method of the prior art. First, by an interruption command of the timer 2c, the detected current data Dc from the sensor 4 is once stored in the memory 2b (S1), and the data Dc is compared with the previously detected and stored data Dp and the differential (=Dc-Dp) is calculated (S2), and absolute value of the differential .DELTA.D (hereinafter, the differential .DELTA.D always represents an absolute value) is compared with the threshold level .DELTA.Q stored in the memory 2b (S3).
If .DELTA.D.gtoreq..DELTA.Q, the CPU 2a transmits the data Dc to the central processing unit 1 as the data showing an appreciable change (S4). Thereafter, the stored previous data Dp in the memory 2b is replaced with the new current data Dc (S5). If .DELTA.D&lt;.DELTA.Q in step S3, no data transmission to the central processing unit 1 is carried out because there is no appreciable change in data.
In the prior art method disclosed using the flow chart of FIG. 2, there still remains a problem. In case of an air conditioning system for temperature control in a building, for example, when the air conditioner is switched on at 8 o'clock in the morning, room temperature to be monitored in the building changes rapidly as shown in FIG. 3. In such case as this, if the prior art method is applied in time zone TZ.sub.a, the chances that the differential .DELTA.D exceeds .DELTA.Q are so often that the data are to be transmitted from the terminal unit 2 to the central processing unit 1 each time of data acquisition. As the result, the central processing unit 1 is subjected to a rush of data and causes an inconvenience of delay in data processing. The similar problem is experienced at the time when the air conditioner is switched off at 6 p.m.
In FIG. 3, it is assumed schematically that the room temperature is to be controlled at 20.degree. C. and the threshold level .DELTA.Q (=.DELTA..tau.) for the room temperature is set at 1.degree. C. (in an actual application, .DELTA..tau. is set at smaller value), room temperature data are transmitted from the terminal unit 2 to the central processing unit 1 six times, i.e., at times t.sub.1, t.sub.2, . . . , t.sub.6, during the time zone TZ.sub.a. There are a large number of terminal units located in other rooms, each of which transmits data in the similar way, therefore, the central processing unit is subjected to a concentrated data rush to transact with during the time zone TZ.sub.a. Data communication state between the terminal unit 2 and the central processing unit 1 is schematically shown in FIG. 4. In FIG. 4, the vertical downward direction shows time progress, and inclined arrows (inclination of the arrow does not include any special meaning) from left to right show that the data is transmitted to the central processing unit 1 at each time when the detected data shows a room temperature rise satisfying the condition of .DELTA.D.gtoreq.1.degree. C. When the room temperature rises at 20.degree. C., the air conditioner is controlled to reduce its function.